At present, most liquid crystal displays employ a PWM mode to regulate an output voltage. The output voltage value of a DC/DC section is determined by different panel manufacturing processes. In design of PWM chip magnitude of voltage, the DC/DC output voltage is usually controlled by regulating an external divider resistor. However, this method has a limited regulation precision and low speed of response. Some of the EEPROM code is directly accessed by the I2C and then converted into a PWM chip feedback voltage by a digital-analog conversion module (DAC) in a PWM chip. Therefore, another set of ROM and digital-analog conversion module (DAC) is added to the PWM chip, which increases the size and cost of the PWM chip.